1. Field of the Invention
The invention relates to a gain controlled amplifier and a cascoded gain controlled amplifier based on the same, and more particularly, to a gain controlled amplifier that can output a gain in linear proportion to a decibel scale through the control of a digital code, and a cascoded gain controlled amplifier that can output a gain having a broadband dynamic range based on the gain controlled amplifier.
2. Description of the Related Art
A gain controlled amplifier (GCA) is a device that can vary an amplification degree thereof in accordance with a control signal. Such a gain controlled amplifier is generally provided in an RF processing circuit of a DVD recorder and is applied to various application fields that require gain compensation. For example, it is used to compensate for a loss of transmission lines or to control a gain that is in linear proportion to a decibel scale.
It is difficult to obtain gain that is in linear proportion to a decibel scale in a CMOS type operational amplifier. Conventional techniques for solving the above drawback will now be explained.
FIG. 1 is a circuit diagram illustrating the construction of an operational amplifier using a resistance network disclosed in U.S. Pat. No. 4,292,596, entitled “Gain Control Circuit” issued to Ishizuka et al., on Sep. 29, 1981.
Referring to FIG. 1, the operational amplifier disclosed in the Ishizuka et al. patent consists of a plurality of switches and resistors, and obtains a gain that is in linear proportion to a decibel scale by adjusting an input/output resistance thereof. However, the operational amplifier has the problems that the whole chip size is increased due to an increase of the resistance and thus the power consumption is also increased. Additionally, since the resistors are floated when the switches are turned off, the circuit becomes in a very unstable state.
FIG. 2 is a circuit diagram illustrating the construction of “Digitally Programmable Decibel-Linear CMOS VGA for Low-Power Mixed-Signal Applications” disclosed in IEEE Trans. Circuit and Syst.—II, May 2000.
Referring to FIG. 2, a CMOS variable gain amplifier (VGA) operates so that an electric current outputted from a current division network (CDN) becomes linear in proportion to a decibel scale. However, the CMOS VGA has the problem that it should employ two amplifiers so as to convert voltage into current and then to convert the converted current into voltage again.